1. Field of the Invention
The present invention relates to a photoelectric conversion device, a production method thereof and an imaging device.
2. Description of the Related Art
As for the solid-state imaging device, there is widely used a flat light-receiving device where photoelectric conversion sites are two-dimensionally arrayed in a semiconductor to form pixels and a signal generated by photoelectric conversion in each pixel is charge-transferred and read out according to a CCD or CMOS format. The conventional photoelectric conversion site is generally formed, for example, by forming PN junction in a semiconductor such as Si.
In recent years, with the progress of a multi-pixel system, the pixel size and in turn, the area of a photodiode part becomes small, and this brings about problems of reduction in the aperture ratio and reduction in the light gathering efficiency. As for the measure to enhance the aperture ratio and the like, studies are being made on a solid-state imaging device having a photoelectric conversion layer (a photoelectric conversion film) using an organic material.
In an organic photoelectric conversion device, it is one of key issues to obtain a high S/N ratio. In order to raise the S/N ratio of an organic photoelectric conversion device, enhancement of photoelectric conversion efficiency and reduction of dark current are required. As to the technique for enhancing the photoelectric conversion efficiency, introduction of a pn junction or a bulk heterostructure in a photoelectric conversion film is studied, and as to the technique for reducing the dark current, introduction or the like of a blocking layer is studied.
In the case of introducing a pn junction or a bulk heterostructure, an increase of dark current often arises as a problem. Also, the improvement of photoelectric conversion efficiency differs in degree according to the combination of materials and in particular, when a method of introducing a bulk heterostructure is employed, the S/N is sometimes not increased from that before introduction of the bulk heterostructure. Thus, it is important what materials are combined.
The kind of material used or the film structure of a photoelectric conversion layer is not only one of main causes for the photoelectric conversion efficiency (exciton dissociation efficiency, charge transportability) and dark current (e.g., dark carrier amount) but also becomes a governing factor of the signal response speed, though this is little referred to in past reports. Particularly, in the case of using an organic photoelectric conversion device as a solid-state imaging device, the matter of importance is to satisfy all of high photoelectric conversion efficiency, low dark current and high response speed, but an organic photoelectric conversion material or a device structure satisfying these performances has not been specifically described so far.
A technique of introducing a bulk heterostructure using a fullerene or a fullerene derivative into the organic photoelectric conversion film so as to bring out high photoelectric conversion efficiency (high exciton dissociation efficiency) is known.
For example, Patent Document 1 discloses a photoelectric conversion layer containing a fullerene or a fullerene derivative. However, further improvement is required in progress of photoelectric conversion efficiency and response speed, and in lowering of dark current.
Also, Patent Document 2 describes a solar cell using a bulk heterofilm composed of a plurality of organic semiconductors, where at least one organic semiconductor is a crystal grain, but this publication has no disclosure about high-speed responsivity or reduction of dark current and is silent on the application to an imaging device using a photoelectric conversion device.
Furthermore, in Non-Patent Document 1, it is suggested that the film structure of a photoelectric conversion layer is important for the elevation of efficiency. However, the photoelectric conversion device of Non-Patent Document 1 is also designed with a solar cell in mind and when the technique described in this publication is directly applied to an imaging sensor, the dark current attributable to the material used or film structure is large, making it impossible to use the device as an imaging device.
[Patent Document 1] JP-A-2007-123707 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
[Patent Document 2] JP-A-2002-076391
[Non-Patent Document 1] Jpn. J. Appl. Phys., 43, L1014 (2004)